It is the purpose of this paper to rediscuss the circumstellar properties of S
stars and to put these properties in perspective with our current understanding
of the evolutionary status of S stars, in particular the intrinsic/extrinsic
dichotomy. This dichotomy states that only Tc-rich ("intrinsic”) S stars are
genuine thermally-pulsing asymptotic giant branch stars, possibly involved in the
M–S–C evolutionary sequence.
Tc-poor S stars are referred to as "extrinsic” S stars, because they are the
cooler analogs of barium stars, and like them, owe their chemical peculiarities
to mass transfer across their binary system.
Accordingly, an extensive data set probing the circumstellar environment of S stars
(IRAS flux densities,
maser emission, CO rotational lines) has been collected and critically
evaluated. This data set combines new observations (9 stars have been observed in the
CO line and 3 in the CO line, with four new detections) with
existing material (all CO and maser observations of S stars published in the
literature). The IRAS flux densities of S stars have been re-evaluated by
co-adding the
individual scans, in order to better handle the intrinsic variability of these
stars in the IRAS bands, and possible contamination by Galactic cirrus.
In the () color-color diagram,
S stars segregate into five distinct regions according
to their Tc content and ZrO/TiO, C/O and IR spectral indices.
Stars with photospheric colors (populating "Region A”) may be
identified with extrinsic S stars. For the other regions characterized
by different excess levels in the 12, 25 and 60 μm bands,
several diagnostics (like the IRAS spectral class, maser emission, and
shape of CO rotational lines) have been collected to infer the physical
properties of the dust shell.
A simple radiative-transfer code has also been used to infer the chemical nature
(carbonaceous or silicate) of the dust grains from the observed IR
colors.
S stars with large excesses and moderate
[25] - [60] excesses (populating Regions B and C) exhibit the signatures of
oxygen-rich shells (9.7 μm silicate emission and SiO maser
emission). The situation is less clear for S stars with small
and moderate indices (populating Regions D
and E). Their IR colors are consistent with carbonaceous grains (as is
their featureless IRAS spectrum, and absence of silicate or SiO maser
emission), but these features may equally well be explained by a detached shell.
For many of these stars with a large 60 μm excess,
the shell is indeed resolved by the IRAS beam at 60 μm.
The prototypical SC star FU Mon is among these. Since SC stars are
believed to be in a very short-lived evolutionary phase where C/O = 1 within 1%,
FU Mon may be a good candidate for the "interrupted mass-loss” scenario advocated
by Willems & de Jong (1988). The CO line profile of FU Mon is
also peculiar in being quite narrow ( km s), suggesting that the mass
loss has just resumed in this star.
Mass loss rates or upper limits have been derived for all S stars observed in the
CO rotational
lines, and range from y-1 for extrinsic S stars to
y-1 (the Mira S star W Aql). These mass-loss rates
correlate well with the color
index, which probes the dust loss rate, provided that y-1. Small mass-loss rates are found for extrinsic S stars,
consistent with their not being so evolved (RGB or Early-AGB) as the Tc-rich S
stars. This result does not support the claim often made in relation with
symbiotic stars that binarity strongly enhances the mass-loss rate.